There has been proposed a projection type image display device including an illuminating system, a reflection type spatial light modulating element illuminated by the illuminating system and a projection optical system (projection lens) that forms an image of the reflection type spatial light modulating element. Using a discharge lamp as the light source of the illuminating system and a liquid crystal element as the reflection type spatial light modulating element, such an image display device is already in the actual use as a relatively large image display device.
The image display device can be produced at a reduced cost by adopting a structure in which a color filter is disposed for each pixel of one reflection type spatial light modulating element and a so-called sequential color display system in which an image is displayed in colors on the time-sharing basis. However, it cannot utilize light with a high efficiency and consumes much power.
The reason why the image display device cannot utilize with a high efficiency is as follows. Since the reflection type spatial light modulating element does not emit light but modulates the polarized state of incident light, it needs a means for splitting a light beam emitted from a light source into polarized components and thereafter synthesizing the split polarized components. Also, different from the self-luminous modulator, the light source emits light even for dark display and there takes place a loss of light corresponding to an efficiency of light utilization that depends upon the open area ratio of the reflection type spatial light modulating element.
In the conventional image display device, an improved efficiency of light utilization can be achieved by optical elements etc. included in the image display device as will be described below.
(Splitting of Light Beam into Polarized Components and Synthesis of Split Polarized Components)
The P-S conversion element is known as a polarization changing element which splits a light beam emitted from a light source of an illuminating system into polarized components and synthesizes the split polarized components. It is disposed between the light source and the reflection type spatial light modulating element. The P-S conversion element is formed by preparing a glass block formed by alternately attaching together glass sheets each having a polarization splitting layer formed from a multilayer film of an inorganic substance and glass sheets each having a reflecting surface formed thereon and cutting the glass block along a plane inclined in relation to the plane of attachment into plates.
A light beam as a mixture of P-polarized light and S-polarized light, incident upon the P-S conversion element, is split by the polarization splitting layer into the P-polarized light and S-polarized light. The P-polarized light and S-polarized light are split by each layer of the P-S conversion element and go out of the P-S conversion element. A half wave (λ/2) plate is disposed at the outgoing side of the P-S conversion element, corresponding to the S- or P-polarized light, whereby a light beam including only either the P- or S-polarized light can be provided.
Using such a P-S conversion element and half wave (λ/2) plate as the polarization splitter, it is possible to improve the efficiency of light utilization of an illuminating system which illuminates a reflection type spatial light modulating element that modulates a polarized component of incident light.
In the illuminating system, a light beam emitted from a light source is reflected by a parabolic mirror for incidence upon the P-S conversion element via a pair of fly-eye lenses. Then, the light beam is passed through the half wave (λ/2) plate and a condenser lens to the reflection type spatial light modulating element.
(Reflection Type Polarization Plate)
The conventional polarization plate allows one of polarized components to pass by while absorbing the other polarized component. However, there is known a reflection type polarization plate that allows one of polarized components to pass by while reflecting the other polarized component, not absorbing it. With the use of the reflection type polarization plate as a polarization changing element, it is possible to utilize the other polarized component by reflecting it again or manipulating it otherwise, thereby improving the efficiency of light utilization.
(Linear Polarization Plate Using Birefringence Multilayer Film)
The linear polarization plate using a birefringence multilayer film is formed by making multilayer lamination of two types of polymer films each having an index anisotropy and different in refractive index from each other and drawing the laminated polymer films. More specifically, the two types of laminated polymer films are coincident in refractive index with each other for one polarization-axial orientation, and different in refractive index from each other for the other polarization-axis orientation. By adjusting the refractive indexes different from each other, it is possible to provide a reflection type polarization plate which allows polarized light having one polarization-axis orientation to pass by while reflecting polarized light having the other polarization-axis orientation orthogonal to the one polarization-axis orientation.
Note that such a reflection type polarization plate is available under the trade name “DBEF” or “HMF” from the 3M.
(Circular Polarization Plate Using Cholesteric Liquid Crystal)
A circular polarization plate utilizing the selective reflectivity of a cholesteric liquid crystal can take the entire visible range as the selectively reflected wavelength band because the cholesteric pitch varies more than 100 nm as disclosed in the Japanese Patent Application Laid Open No. 281814/1994 for example. With the use of such a cholesteric circular polarization plate, it is possible to provide a circular polarization plate having no wavelength dependence.
A circular polarization plate using the cholesteric liquid crystal and a polarization changing element using the circular polarization plate are disclosed in the Japanese Patent No. 2509372 for example. The invention set forth in this Japanese Patent utilizes the fact that with the characteristic of the circular polarized light, that is, a change by 180 deg. caused by one reflection, clockwise circular polarized light changes to counterclockwise circular polarized light, and vice versa.
A combination of the cholesteric liquid crystal with a reflecting mirror provides a polarization splitting synthesizer. The polarization splitting synthesizer using the above-mentioned linear polarization needs a half (λ/2) plate, but the polarization splitting synthesizer using the circular polarization needs not any half (λ/2) plate.
That is, a light beam emitted from the light source is either incident directly upon the cholesteric liquid crystal via a condenser lens or reflected by the reflecting mirror and then incident upon the cholesteric liquid crystal depending upon the outgoing direction of the light beam from the light source.
At this time, the circular polarized light in one direction passes by the cholesteric liquid crystal while the circular polarized light in the other direction is reflected by the cholesteric liquid crystal. Thus, the circular polarized light in the other direction, reflected by the cholesteric liquid crystal, is reflected by the reflecting mirror to be the circular polarized light in the one direction which will be incident again upon the cholesteric liquid crystal and pass by the latter. Each light passing by the cholesteric liquid crystal becomes the circular polarized light in the one direction.
The image display device using the aforementioned illuminating system have the following problems to solve:
The aforementioned illuminating system can only be produced through a complicated process of production. Thus, it is thus complicated to produce and expensive.
The circular polarization plate disclosed in the aforementioned Japanese Patent Application Laid Open No. 281814/1994 has no wavelength dependence but has no sufficient polarization splitting characteristic. On this account, it is necessary to use a combination of the circular polarization plate with a light absorption type polarization plate (which absorbs other polarized light) in order to assure a contrast required for the image display device. Therefore, it is difficult for the conventional image display device to have an improved efficiency of light utilization.
In the illuminating systems disclosed in the aforementioned Japanese Patent No. 2509372 and Japanese Patent Application Laid Open No. 281814/1994, the shape of a reflector used for the discharge lamp actually used as a light source or illumination of the reflection type spatial light modulating element by reflected light from the reflector is not so effective for an improved efficiency of light utilization as expected.
More specifically, since the shape of the actual reflector for the discharge lamp is a paraboloid or ellipsoid of revolution, so a light beam reflected by the polarization splitting element formed of a cholesteric liquid crystal to the light source is reflected twice by the reflector. When the light beam is reflected twice with the phase change caused by one reflection by the reflector being 180 deg., the phase will not be changed.
Further, since the reflector reflects the P-polarized light and S-polarized light with a difference in reflectance between them and these polarized rays of light are changed in phase and scattered due to passing by the glass tube of the discharge lamp as the light source, the effect of polarization changing will be lower. Also, in case the reflector is a parabolic mirror, a light beam emanated from a focal point will return to the focal point when it is reflected by the reflection type polarization plate, but a light beam emanated from any other point than the focal point will not always return to the point from which it has being emanated when it is reflected by the reflection type polarization plate.
Also, in case a spheroidal mirror is used as the reflecting mirror, light reflected by the cholesteric liquid crystal will be absorbed by the electrodes of the discharge lamp or diverged after reflected by the reflecting mirror, without returning to the light emission point of the light source, depending upon where the cholesteric liquid crystal is positioned. The “diversion” will increase the etendue of the light source, which will cause the efficiency of illumination-light utilization to be reduced.
As above, being disadvantageous in the efficiency of light utilization and manufacturing cost, the polarization changing element in the conventional illuminating system cannot return light to its source with a sufficient efficiency.
Also, in case the reflection type polarization plate has to be increased in area, the optical parts included in this illuminating system will be very expensive. Further, the light returned to the light source will possibly cause the illuminance to be nonuniform at the reflection type spatial light modulating element.
In addition, a part, made unnecessary for image display, of the illumination light having arrived at the reflection type spatial light modulating element may be returned to the light source and reused for illuminating the reflection type spatial light modulating element.
In this case, since the light emanated from the light source has arrived at the reflection type spatial light modulating element via a pair of fly-eye integrators for example, it will go back again to the light source through the fly-eye integrators. For reflecting the light having returned to the light source and directing it again toward the reflection type spatial light modulating element, the optical axis of an optical system located downstream of the fly-eye integrators has to be displaced in relation to that of the light source.
At this time, in case the optical axis of the optical system downstream of the fly-eye integrators is displaced only in one of longitudinal and lateral directions perpendicular to the optical axis, it is difficult to sufficiently reflect the light having returned to the light source.
Also, in case all the optical parts such as the projection optical system etc. located downstream of the fly-eye integrators are displaced in relation to the optical system including the light source and extending to the fly-eye integrators, all the optical systems will not possible be in line with each other. In this case, a support mechanism to support the optical systems will be extremely complicated and include many parts. Especially, the fly-eye integrators, condenser lens, field lens, etc. have to be aligned with each other with a high precision and thus the precision required for the support mechanism will be very high. The support mechanism will be difficult to produce.